A) Field of the Invention
The present invention relates to an optical semiconductor device having a diffraction grating, and more particularly to an optical semiconductor device having a vertical groove diffraction grating.
B) Description of the Related Art
The following methods are known as a method of forming a diffraction grating of a distributed feedback (DFB) laser or a distributed Bragg reflector (DBR) laser: (1) a regrowth method by which a diffraction grating pattern is formed on the surface of a lower growth layer, and an upper crystal growth layer is regrown on the lower growth layer to form a diffraction grating at the interface of the growth layer; and (2) collective etching method by which a diffraction grating is formed by etching vertical grooves periodically disposed along a waveguide direction on the sidewalls of a waveguide structure, etching starting from the surface of epitaxial stack structure. It is necessary for the regrowth method (1) to perform crystal growth twice, which is likely to increase cost. If the surface of the lower growth layer is made of material which contains Al, the regrowth itself may become not easy. For applications requiring low cost, the collective etching method (2) is preferable in many cases.
Oku et al.: IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 3, p. 682, 1999 reports the characteristics of a DBR laser and a DFB laser formed by etching stacked layers including an InGaAs—GaAs strain quantum well active layer to a depth of 700 nm and 800 nm at a pitch of 150 nm, to form a DBR laser and a DFB laser having an oscillation frequency of 980 nm, as compared to the characteristics of a Fabry-Perot type laser.
It can be said that the collective etching method (2) is a simple manufacture method which can form at the same time both a waveguide structure and a diffraction grating by deeply etching a stacked layer structure. However, if the width of the waveguide is made narrow to some extent, a higher order transverse mode becomes likely to oscillate, forming two or more optical intensity peaks along a waveguide width direction. As the waveguide width is narrowed in order to suppress a higher order transverse mode, a device resistance increases.
JP-A-2003-152273 proposes to suppress a higher order transverse mode by giving a loss by covering a diffraction grating with an absorbing material such as GaAs. This method gives a loss also in the fundamental mode, being likely to raise a threshold value and lower an efficiency.
JP-A-2001-133647 proposes to eliminate a higher order mode by forming a diffraction grating on sidewalls of a waveguide structure at a pitch longer than λ/2 and shorter than λ, where λ is a waveguide effective wavelength. This diffraction grating has a function different from the diffraction grating of a DFB laser or DBR laser.